The entry says that the instrument nobody read on the night of 27 January 1986 was the full set of flights, including the ones that came back clean. That claim has been read four ways since, by people who all had the same twenty-four flights in front of them. This piece sets the readings side by side, because the differences between them are the complication of the story, and the entry could only summarise them.

The Commission, June 1986

The Commission’s chapter on the history ends with a section headed “Temperature Effects” and two charts. The text: “The managers compared as a function of temperature the flights for which thermal distress of O-rings had been observed, not the frequency of occurrence based on all flights. In such a comparison, there is nothing irregular in the distribution of O-ring ‘distress’ over the spectrum of joint temperatures at launch between 53 degrees Fahrenheit and 75 degrees Fahrenheit. When the entire history of flight experience is considered, including ‘normal’ flights with no erosion or blow-by, the comparison is substantially different.” Figure 6 plots the seven flights with field-joint distress against calculated joint temperature; Figure 7 adds the seventeen with none. “Only three incidents of O-ring thermal distress occurred out of twenty flights with O-ring temperatures at 66 degrees Fahrenheit or above, whereas, all four flights with O-ring temperatures at 63 degrees Fahrenheit or below experienced O-ring thermal distress. Consideration of the entire launch temperature history indicates that the probability of O-ring distress is increased to almost a certainty if the temperature of the joint is less than 65.” The finding: “A careful analysis of the flight history of O-ring performance would have revealed the correlation of O-ring damage and low temperature. Neither NASA nor Thiokol carried out such an analysis.”

Two things about the charts need saying. The temperatures on their horizontal axis are “calculated joint temperature”, and the Commission’s own table of flights carries them with the note “mean calculated (±5°F)”. They were computed after the accident, by the investigation, from weather records and models of how the steel and propellant took up and gave off heat; the figures the engineers had on 27 January were two of the same kind, 53 degrees for flight 15, calculated in February 1985, and 75 for flight 22, set beside the air temperatures on one chart. And the Commission counted the same history a second way in its chapter on the cause, by air temperature at launch, which anyone could have looked up: “Of 21 launches with ambient temperatures of 61 degrees Fahrenheit or greater, only four showed signs of O-ring thermal distress; i.e., erosion or blow-by and soot. Each of the launches below 61 degrees Fahrenheit resulted in one or more O-rings showing signs of thermal distress.” That count uses the air at the pad rather than the calculated temperature of the joint, and a different cut-off, which is why its numbers differ from the charts’. Either way of counting gives the same answer.

The House committee, October 1986

The House Committee on Science and Technology accepted the cause and most of the findings, and then said: “The Committee does agree that the Marshall Space Flight Center should have passed along to higher management levels the temperature concerns that Thiokol engineers raised the night before the launch of Mission 51-L. However, the Committee feels that the underlying problem which led to the Challenger accident was not poor communication or inadequate procedures as implied by the Rogers Commission conclusion. Rather, the fundamental problem was poor technical decision-making over a period of several years by top NASA and contractor personnel, who failed to act decisively to solve the increasingly serious anomalies in the Solid Rocket Booster joints.” Information on the joint “was widely available and had been presented to all levels of Shuttle management … No one suggested grounding the fleet, nor did NASA embark on a concerted effort to remedy the deficiencies in O-ring performance. Rather, NASA chose to continue to fly with a flawed design and to follow a measured, 27-month, corrective program.”

The committee’s own hearings produced the hardest fact in the case against the engineers. Boisjoly told it that after January 1985 the temperature concern “wasn’t pursued any further” because “it was nobody’s expectation we would ever experience any cold weather to that degree before we had a chance to fix it again.” Congressman Bill Nelson then pointed out that flight 61-C had been scrubbed four times in the weeks before it flew on 12 January 1986, “when the temperatures were less than 53 degrees during several of those scrubs, reaching down into the low 40s during the first scheduled launch”, and asked whether the same concerns had come up. McDonald: “I am not aware that they had, Congressman. I don’t know. I wasn’t at that launch, but I don’t recall that that came up.” The commander of 61-C, Robert Gibson, recalled no discussion either. Congressman Ron Packard asked McDonald whether Challenger’s own earlier attempts had been below 53 degrees. “That is correct.” Had temperature been discussed? “No, it was not … Nowhere was it, no.” Why not? “I don’t, I can’t answer that.” Kilminster, asked why the evaluation waited until the night before, said it had been in response to a specific request from NASA. The committee concluded: “The Committee finds no evidence that new data were presented during the January 27th teleconference that were not available to Thiokol at the time of the Flight Readiness Review … The Committee finds that Thiokol’s advice and recommendations to NASA were inconsistent, and therefore, the arguments presented during the January 27th teleconference might not have been as persuasive at the time as they now appear to be in hindsight.” It also noted, in a footnote, that the papers of two Marshall engineers on the line that night, Wilbur Riehl and John Miller, showed that “some of the Marshall engineering staff shared the concerns expressed by Thiokol engineers.”

The statisticians, 1989

In December 1989 the Journal of the American Statistical Association published “Risk Analysis of the Space Shuttle: Pre-Challenger Prediction of Failure”, by Siddhartha Dalal and Edward Fowlkes of Bellcore and Bruce Hoadley of Navesink Research. They began from the chart the engineers had used and the Commission’s sentence that a careful analysis would have revealed the correlation, and set out “to give more substance to this quote and show how statistical science could have provided valuable input to the launch decision process.” Using the twenty-three flights before Challenger with recovered boosters, they fitted logistic regressions of thermal distress against joint temperature and found “strong statistical evidence of a temperature effect on incidents of O-ring thermal distress.” Their estimate, from a model of how a distressed ring becomes a failed joint, was that “a probabilistic risk assessment at 31°F, the temperature at which Challenger was launched, yields at least a 13% probability of catastrophic field-joint O-ring failure. Postponement to 60°F would have reduced the probability to at least 2%.” They noted that the engineers’ chart left off the flights with zero incidents “because it was felt that these flights did not contribute any information about the temperature effect”, and that “clearly, the key question was What would have constituted proof that it was unsafe to launch?” The article’s claim was that statistical science could have supplied what the night lacked: an estimate of the risk, with its uncertainty. Its data set has since become a standard teaching example in courses on logistic regression.

The critics and the engineers, 1997 to 2002

In Visual Explanations (1997) Edward Tufte reproduced the thirteen charts and argued that they failed to show the one thing the engineers were claiming, a relation between temperature and damage; that the Blow-by History chart, by naming only flights 15 and 22, invited the “rhetorically devastating” comparison between a cold flight and a warm one; that 92 per cent of the temperature data were missing; and that a single scatterplot of damage against temperature for all flights would have made the risk undeniable. His verdict on the chart makers was that they “didn’t quite know what they were doing, and they were doing a lot of it.” The book is in copyright, and these phrases are known here through the reply to it.

The reply came from Wade Robison, a philosopher, with Boisjoly and two others, first as a lecture in 1996 and then in Science and Engineering Ethics in 2002. It makes three points of fact. First, the engineers did not have the temperatures Tufte’s plot requires: “they did not know the temperatures even though they did try to obtain that information.” Temperature had not been thought a relevant variable and had not been collected as a matter of course; in September 1985 Boisjoly had listed the data the task force needed, including temperatures, and “the engineers received none of the data they requested.” Second, an air temperature is not an O-ring temperature. Flight 15 launched in air at 66 degrees with its rings calculated at 53; flight 22 in air at 78 with its rings later calculated at 75; and Tufte’s horizontal axis, the authors say, mixes the two kinds. Third, the engineers’ concern was not erosion but blow-by, a ring that fails to seal at all, which is what the cold does to a stiff ring, and Tufte’s vertical axis measures the wrong thing. On the night, they argue, the engineers were not making a statistical inference from two points but a conservative one from a flight that was, in Tufte’s own words, “completely outside the engineering database accumulated in 24 previous flights”, and the burden of proof was reversed on them. The paper’s closing judgement is on Tufte: “it is not competent, and is morally wrong, to design a criticism that so badly misrepresents the position of those one is critiquing.”

Diane Vaughan’s The Challenger Launch Decision (1996) belongs beside these. She went to the National Archives expecting “a typical case of misconduct” and found, she said, “something completely different”: a decision structure NASA followed consistently, engineers who could learn what the rings had done only after a flight came back, and a history in which each fix seemed to work, so that erosion came to be treated as an acceptable risk. The book argues that no rule was broken, and that what happened was socially organised; her name for it, the normalisation of deviance, was invoked again after Columbia, and she sat on the board that investigated that loss. As she wrote: “The Challenger disaster was an accident, the result of a mistake. What is important to remember from this case is not that individuals in organizations make mistakes, but that mistakes themselves are socially organized and systematically produced.”

What the four agree on

The Commission says the analysis could have been done and was not. The committee says the engineers themselves had not asked for 53 degrees until NASA asked them, days after launching and scrubbing below it. The statisticians say the answer was in the twenty-three flights at 13 per cent and could have been computed before the night. The engineers’ defenders say the joint temperatures were not theirs to compute, that the air temperatures would have been the wrong number, and that two points and a bench test were enough for a conservative recommendation that their managers accepted and then withdrew under pressure. All four agree on the fact the entry rests on: that the flights with nothing wrong were left off the table, by everyone, until a commission put them back.

Sources

• Presidential Commission on the Space Shuttle Challenger Accident, Report to the President, volume 1, June 1986: Chapter VI, “Temperature Effects” and Figures 6 and 7, with the table of flights and its note on calculated temperatures; Chapter IV, finding 12, for the count by ambient temperature. chapter 6, chapter 4
• US House of Representatives, Committee on Science and Technology, Investigation of the Challenger Accident, House Report 99-1016, 29 October 1986: the summary of differences with the Commission; Issue 4 on the scrubs of 61-C and 51-L and the finding on inconsistency; the footnote on Riehl and Miller. govinfo.gov
• Siddhartha R. Dalal, Edward B. Fowlkes and Bruce Hoadley, “Risk Analysis of the Space Shuttle: Pre-Challenger Prediction of Failure”, Journal of the American Statistical Association 84 (408), December 1989, pages 945 to 957. tandfonline.com
• Wade Robison, Roger Boisjoly, David Hoeker and Stefan Young, “Representation and Misrepresentation: Tufte and the Morton Thiokol Engineers on the Challenger”, Science and Engineering Ethics 8 (1), 2002, pages 59 to 81; the conference text, from which the quotations here and the quotations from Tufte are taken, is on the Online Ethics Center. onlineethics.virginia.edu
• Paul Hond, “How the Challenger Disaster Became a Case Study of the ‘Normalization of Deviance’”, Columbia Magazine, winter 2025 to 2026, for Diane Vaughan’s account of her research and the sentence quoted from her book. magazine.columbia.edu